1. Field of the Invention
The present invention relates generally to a Group III nitride substrate (a substrate containing Group III nitride crystals), a method of manufacturing the same, and a semiconductor device.
2. Related Background Art
A Group III nitride compound semiconductor such as, for instance, gallium nitride (GaN) (hereinafter also referred to as a “Group III nitride semiconductor” or a “GaN-based semiconductor”) has been gaining attention as a material for semiconductor elements that emit blue or ultraviolet light. A blue laser diode (LD) is used for high-density optical disk devices or displays while a blue light emitting diode (LED) is used for displays, lighting, etc. It is expected to use an ultraviolet LD in the field of biotechnology or the like and to use an ultraviolet LED as an ultraviolet source for a fluorescent lamp.
Generally, substrates of a Group III nitride semiconductor (for example, GaN) that are used for LDs or LEDs are formed through vapor phase epitaxy. For instance, substrates have been used that are obtained through heteroepitaxial growth of Group III nitride crystals on a sapphire substrate. However, the sapphire substrate and GaN crystals are different from each other in lattice constant by 13.8% and in coefficient of linear expansion by 25.8%. Hence, a sufficiently high crystallinity cannot be obtained in the GaN thin film obtained through the vapor phase epitaxy. Generally, crystals obtained by this method have a dislocation density of 108 cm−2 to 109 cm−2 and thus the reduction in dislocation density has been an important issue. In order to resolve this issue, efforts have been made to reduce the dislocation density and thereby, for example, an epitaxial lateral overgrowth (ELOG) method has been developed. With this method, the dislocation density can be reduced to around 105 cm−2 to 106 cm−2, but its manufacturing process is complicated.
On the other hand, besides the vapor phase epitaxy, a method of carrying out crystal growth from the liquid phase also has been studied. However, since the equilibrium vapor pressure of nitrogen is at least 10000 atm at a melting point of a Group III nitride single crystal such as, for instance, GaN or AlN, conventionally it has been understood that a condition of 8000 atm at 1200° C. is required for growing GaN from liquid phase. In this connection, recently, it was made clear that GaN was able to be synthesized at relatively low temperature and pressure, specifically, 750° C. and 50 atm, by using a Na flux.
Recently, single crystals with the maximum crystal size of about 1.2 mm are obtained by a method in which a mixture of Ga and Na is melted in a nitrogen gas atmosphere containing ammonia at 800° C. and 50 atm, and then the single crystals are grown for 96 hours using the melt (for instance, JP2002-293696A).
Furthermore, another method also has been reported in which after a GaN crystal layer is formed on a sapphire substrate by a metal organic chemical vapor deposition (MOCVD) method, single crystals are grown by a liquid phase epitaxy (LPE) method.
However, in order to manufacture semiconductor devices with excellent characteristics at low cost, it has been required to provide a method of manufacturing a Group III nitride substrate with a lower dislocation density than that of conventional one and a method of manufacturing a Group III nitride substrate at lower cost. A method of growing nitride crystals from liquid phase is expected to be a method that allows a Group III nitride substrate with less defects to be obtained. However, considerable variations in crystallinity are caused depending on a seed crystal to be used and on in-plane carrier concentration of the nitride substrate. Furthermore, the growth rate is not so high, which has been a problem.